As one of the Tokugawa family's foremost military commanders, he was considered one of its "Four Guardian Kings" (shitennō 四天王) along with Sakai Tadatsugu, Honda Tadakatsu and Ii Naomasa.

However, until around April of the first year of the Genki era, diplomatic negotiations were being conducted between Yasumasa and Tsuchiya Masatsugu, who acted as intermediaries between the two sides.

[citation needed] Immediately after the Nagashino battle, he and Tadakatsu captured the Suwahara Castle, and was awarded by Ieyasu a famous sword which crafted by Takagi Sadamune.

[14][15] Later in 1581, Yasumasa were recorded has stormed Tanaka castle which guarded by Takeda clan general named Ichijō Nobutatsu.

[a] In 1581, Yasumasa participated in the Tokugawa army attack on Tanaka castle which guarded by Takeda clan general named Ichijō Nobutatsu.

In this battle, Yasumasa fought together with Matsudaira Ietada, Honda Tadakatsu, and Ii Naomasa as they all climbed to Tanaka castle wall and fighting Nobutatsu's soldiers.

[16] In 1582, after the Honnō-ji Incident, Yasumasa accompanied Ieyasu in an arduous journey to escape the enemies of Nobunaga in Sakai and returning to Mikawa.

However, the Hōjō clan in the Kantō region also led an army of 55,000 men and crossed the Usui Pass to invade Shinano Province.

However, Yasumasa and Yasutaka advance were held back by reinforcement forces led by Hori Hidemasa, thus they retreat from the battle.

[37] Hideyoshi were reportedly furious when hearing this and issuing order that he will reward of 100,000 Kan (Japanese gold ingot) to anyone who could bring him the head of Yasumasa.

Tadatsugu, Okanabe Mori, and Yamaguchi Shigemasa spearheading the attack towards Shimojima castle, while Sakakibara Yasumasa, Osuga Yasutaka were deployed to capture any fleeing defenders.

Later, he also appointed two former Takeda vassals, Naruse Masakazu and Okabe Masatsuna, as magistrates under authority of Ii Naomasa and Honda Tadakatsu, while he also ordered all of former Takeda vassals who now serve him to impart any military doctrines and structures they knew during their service under Takeda clan.,[45] and lastly, he ordered the three of his prime generals, the so-called "Tokugawa Four Heavenly Kings," Ii Naomasa, Honda Tadakatsu, and Sakakibara Yasumasa, to serve as supreme commander of this new military regiments.

[47] Then in following month, the three of them joined by Sakai Tadatsugu to accompany Ieyasu in his personal trip to Kyoto, where the four of them became famous as Tokugawa-Shitennō(Tokugawa's Four Guardians".

Furthermore, Ieyasu viewed Yasumasa needed more manpowers to manage vast domains, so he transfer three of other senior retainers Nagashige Nakane, Tanemasa Harada, and Katsushige Murakami to join the administration team.

[5] In 1591, Ieyasu Tokugawa brought Yasumasa, Honda Tadakatsu, and Ii Naomasa to participate in the suppression of Kunohe rebellion.

[52][53] In 1598, after the death of Hideyoshi Toyotomi, Yasumasa mobilize a Tokugawa army from Kanto and marched to Seta in Ōmi Province on the orders from Ieyasu.

In the end, Yasumasa were ordered to leave the matter to Ieyasu himself who successfully reconcile the case and averted the civil war between two faction.

However, the aftermath of this incident causing many of Hideie retainers such as Sakazaki Naomori to change their allegiance into the master of Yasumasa, which is Ieyasu Tokugawa.

[56] In July 27, Yasumasa sent a letter to Akita Muneki informing him that Ishida Mitsunari and Otani Yoshitsugu had "parted ways," and that Yodo-dono, the three Toyotomi magistrates (Maeda Gen'i, Masuda Nagamori, and Nagatsuka Masaie), and Maeda Toshinaga had requested Ieyasu to come to Kyoto, and that command over the Aizu region had been transferred from Ieyasu to Hidetada.

[61] After the death of Ieyasu, the original members of Yasumasa administration team, Nagashige Nakane, Tanemasa Harada, and Katsushige Murakami, seeking permission to leave their posts and return to serve the Tokugawa clan directly as Hatamoto.

[64][65] Kazuto Hongō, history professor of University of Tokyo, gave his assessment that Yasumasa was an example of particularly talented general of Tokugawa clan who also possess an expertize in administration.

[67] Alternatively, modern era popular theories regarding the meaning of Yasumasa's choice of the character, which though as "Entering the Battlefield with clear mind", or "Serve their master without selfish desire".

However, Harold Bolitho pointed out that this is instead their own decision to not involve themselves in Shogunate administrations and rather focusing on governing their own respective military domains.

[70] By appointed as Rōjū along with Ōkubo Tadachika and Honda Masazumi, Yasumasa held one of highest position in Tokugawa shogunate.